1. Field of the Invention
This application relates to detecting fault in transmission lines with shunt reactors, in particular, to the fault clearance detection methods and systems in transmission lines with shunt reactors. When fault occurs on a transmission line with shunt reactors, circuit breakers on both ends open to isolate the fault from rest of power system, however, the shunt reactors stay connected to the line. Once, the fault is cleared, circuit breakers can be reclosed for normal operation. This invention describes the methods and apparatuses to detect the fault cleared state in wide variety of faulty types and conditions, which can be used for reclosure.
2. Description of the Related Art
FIG. 1 elucidates the schematic of a transmission line with two end shunt reactors system. Faults on a transmission line occur due to man-made and/or natural causes. These faults can be temporary or transient which extinguish (or clear) themselves after a certain period (ms to sec). The faults can be permanent which would need longer time to clear (hours to days) through manual intervention by line crew. Since majority of transmission line faults are temporary, fixed time delay based automatic reclosure techniques are currently employed in many utilities to re-energize the transmission line with and without shunt reactors to increase the reliability of transmission system. IEEE Guide for “Automatic Reclosing of Line Circuit Breakers for AC Distribution and Transmission Lines”, IEEE C37.104-2002, April 2003 and Chapter-14 of Alstom Inc., “Network Protection and Automation Guide”, 2011 outline several design considerations for implementing automatic reclosure for transmission lines.
Different transient characteristics are expressed in a shunt reactor compensated transmission line during fault existence and after the fault clearance. These characteristics are highlighted in FIG. 2 using faulty phase voltage measurements. The techniques used to detect the fault clearance on transmission line without shunt reactors are not effective in transmission line with shunt reactors. Taking advantage of ringing (or beat frequency) phenomenon expressed in such transmission lines; researchers have proposed several techniques to distinguish temporary and permanent faults. The ringing phenomenon occurs due to exchange of energy between line capacitances and shunt reactors. The voltage built on a transmission line after fault is cleared is referred as recovery voltage. An adaptive reclosure technique can be considered in different ways based on the detection. Three ways of detections are:
1) The technique detecting the instant of fault clearance by analyzing transients;
2) The technique detecting the instance of fault clearance by matching the specific signatures expressed in the transients of faulty phase of transmission line;
3) The technique detecting the distinguishing characteristics expressed by the transmission line during temporary and permanent faults.
The same algorithms can be used in all the above three defined detection approaches. Conversely, a detection algorithm to identify the clearance of fault can also be interpreted to identify the existence of temporary/permanent fault for achieving adaptive reclosure. Adaptive reclosure algorithms utilize shunt reactor currents and terminal voltage measurements to identify the transient and permanent faults. Adaptive reclosure can be implemented in two scenarios:                1) All three phases of transmission line trip to isolate the fault and three phases can be adaptively energized after clearance of the fault.        2) In case of single line to ground faults, only faulty phase is tripped keeping the other two healthy phases energized. The fault phase is adaptively energized after clearance of the fault. This operating mechanism is referred as Single phase adaptive reclosing (SPAR).        
The prior art in adaptive reclosure can be understood by classifying detection techniques into the below mentioned categories:
Recovery Voltage based Methods:
In US patent, publication number U.S. Pat. No. 7,317,599 B2, fault clearance in a shunt compensated transmission line is detected and applied for SPAR by measuring the frequency content when the recovery voltage goes beyond certain limit (nominal value). Even though, this approach is effective in a single-phase (or line) to ground fault (SLG), but it is not applicable in other kinds of faults with three phase opening.
In another US patent publication number US20120176712 A1, even to odd harmonic ratio has been used to detect the fault clearance state. However, harmonic ratio will not be effective in transmission systems having multiple frequency components in recovery state and fault dependent damping factors.
In IEEE Bologna Power Tech Conference Proceedings, 2003, a publication on “750 kV reactive power control, automatic reclosing and overvoltage protection”, proposed single phase and three phase reclosing by sensing the extinction transients in the line voltage by means of special filters. The reclosure command is initiated when the recovery voltage in the faulty phase exceeds the set limit.
At 3rd International Conference on Electric Utility Deregulation and Restructuring and Power Technologies, a publication on “Power spectrum of fault phase voltage based single-phase adaptive reclosure” proposed SPAR operation based on the detection of frequency components in the recovery voltage which consist of power frequency and resonant frequency components in temporary faults, and only have power frequency components in permanent faults.
Pattern matching/Estimation approaches
Pattern Matching Method:
A European Patent publication number EP2319151 A1 (US 20110148430 A1) describes the methodology to detect a temporary or a permanent fault by comparing the transients in a transmission line with the pre-determined pattern. A threshold value has been used for wide-varying fault types, fault resistances and fault durations to compare the degree of similarity. The pattern matching technique will not be effective in those fault conditions when one or more frequency components during recovery period are damped.
Parameter Identification Methods:
Another publication at 3rd International Conference on Electric Utility Deregulation, Restructuring and Power Technologies on “Study on single-phase adaptive reclosure scheme based on parameter identification” used instantaneous shunt reactor current and voltage measurements to determine resistance, inductance and capacitance parameters of the transmission lines. Comparing the continuously calculated shunt and neutral inductances against actual values served to distinguish temporary and permanent faults. The proposed SPAR technique needs appropriate scaling factors to account for modeling errors resulting from model simplification, sampling precision and oscillating components.
Similar scheme has been utilized by another publication on “Permanent Faults Identification for Three-phase Autoreclosure on Transmission Lines with Shunt Reactors” at International Conference on Advanced Power System Automation and Protection, 2011 to estimate the parameters of shunt and neutral inductances but, a zero modal circuit has been utilized for calculations and the paper proposed the reclosure application on phase-phase fault.
Voltage/Current Estimation Methods: SPAR Techniques
Voltage or current through the neutral or shunt reactor is calculated using transient fault model. The comparison of calculated current versus measured current has been used to determine the nature of fault. Difference between the estimated and calculated voltages across shunt reactor on double circuit line has been utilized to detect a temporary versus permanent fault in the paper “Fault nature identification for single-phase adaptive reclosure on double circuit Extra High Voltage (EHV) transmission lines with shunt reactors” published in International Conference on High Voltage Engineering and Application, 2010.
Similarly, current estimation and comparison through difference, ratio and scaling factor calculations have been utilized to identify the fault nature in the paper titled “A Novel Single-Phase Adaptive Reclosure Scheme for Transmission Lines With Shunt Reactors” published in IEEE Transactions on Power Delivery, April 2009.
Reactor Current Methods:SPAR Techniques
Shunt reactor current has low frequency and power frequency components in a single phase operation in transient faults, however permanent faults have only power frequency and decaying dc components. An energy function calculated through differences in current samples between two time instants has been used to eliminate the fundamental and dc components and retain the low frequency component in order to detect nature of fault in “Single-Phase Adaptive Reclosure of EHV Transmission Lines Based on Shunt Reactor Current Identification” published in Power and Energy Engineering Conference, Asia-Pacific, 2009.
A similar dual window energy ratio method has been proposed in paper “A Dual-Window Transient Energy Ratio-Based Adaptive Single-Phase Reclosure Criterion for EHV Transmission Line” published in”, IEEE Transactions on Power Delivery, 2007.
A Prony signal analysis method which fits the sampled data of reactor current into set of linear exponentials has been utilized in publication “New Algorithm for Adaptive Single-phase Reclosure on EHV Transmission Lines” in Power and Energy Engineering Conference, Asia-Pacific, 2011 to identify the low frequency component characteristics in a transient fault to initiate reclosure.
Comparison of ratio of amplitude of low frequency component and power frequency component of the current on the open phase has been utilized to distinguish permanent faults from transient faults in paper “Study on Free-oscillation Components Characteristics and Single-phase Adaptive Reclosure in Reactored Transmission Lines” published in Power and Energy Engineering Conference, Asia-Pacific, 2010.
Comparison of summation and differences in the current measured in either ends of shunt reactors has been used to identify single phase permanent fault and block reclosure in paper “Single-Phase Permanent Fault Detection for Reactored EHV/UHV Transmission Lines” published in”, Power and Energy Engineering Conference, Asia-Pacific, 2011. Threshold and scaling factors have been utilized in decision making.
Characteristic analysis of current in the shunt reactors and voltage on the open phase has been performed in paper “Single-phase Adaptive Reclosure for EHV and UHV Transmission Lines with Shunt Reactors” published in Power and Energy Engineering Conference, Asia-Pacific, 2010. The paper proposed the comparison of the ratio of maximum and minimum envelope line of the ringing waveform of shunt reactor and line voltage against the thresholds to identify transient and permanent faults. Due to higher max/min ratios in reactor currents than terminal voltage measurements, the current based criterion was proposed as a primary method of detection and voltage based criterion as a backup.
The traditional fixed time delay based reclosure approaches are not reliable and result in unnecessarily prolonging the reclosure in case of temporary faults, or unsuccessful reclosure attempts in case of permanent fault which adversely impacts the life of substation and transmission line equipment and also negatively affects the system stability.
Many of the proposed adaptive reclosure methods utilize thresholds and multiplication factors to identify the clearance of secondary arc and to differentiate transient versus permanent faults. There is no direct method to calculate these factors; determination of scaling factors may need an extensive study or broad experience on wide variety of faults. Thus, these factors are set to a higher value to be conservative and reliable in order to preclude false-positive results. This leads to de-sensitization of detection algorithm and adversely impact the results in 2 ways:
The reclosure may get delayed until the observed parameter goes above threshold value. Incorrectly interpret transient fault as permanent and block reclosure in case of transient faults that fall below threshold.
Even though, some adaptive reclosure techniques proposed in the paper use transmission line parameters in adaptive reclosure detection algorithms, there is no explanation on the length of window that needs to be used in time domain for calculations and detection. This information determines the efficacy of an algorithm under different fault conditions. An arbitrary window length will not be able to capture the transients to integral multiple cycles and will result in wide variations in frequency especially at smaller window lengths.
Some of the adaptive reclosure techniques utilize lumped circuit representation model to identify parameters. This approximation leads to inaccuracies in estimation and thus requires scaling factor to account for differences. Any usage of scaling factor results in de-sensitizing the detection algorithm as mentioned above.
All the proposed adaptive reclosure techniques are applicable only in case of single phase operation and single line to ground faults except for one paper which proposed three phase reclosure on SLG fault and another paper illustrated the technique on line-line fault. None of the approaches are applicable universally for all kinds of faults namely, SLG, two phase/line to ground fault (2LG), three phase/line to ground fault (3LG), phase-to-phase/line-line fault (LL or 2L) and single phase adaptive reclosing (SPAR).
Some of the adaptive reclosure techniques which depend on amplitude of the recovery voltage/current are sensitive to longer fault durations as the much of the energy in the transmission line is dissipated in the fault resulting in smaller amplitude of recovery voltage and currents. The same limitation also applies depending on the instant of fault on the sine wave. The fault current is inductive in nature and lags voltage by 90°. The initial conditions and the energy contained in faulty/healthy phase depend on the instant of the fault.
Due to above limitations, the proposed adaptive reclosure techniques are not reliable and easy to implement and hence not a single method has a wide implementation across power industry. In order to increase the reliability of these techniques, some researchers have considered further augmenting the decision process with additional statistical and neural network techniques. However, these kinds of techniques need lot of previous history fault records for training purposes which is practically not feasible making them difficult to implement.
Summarizing the current state of art on fault clearance detection, the requirements of an effective and efficient adaptive reclosure method are that, it needs to be applicable under various types of fault conditions, provide fast response, easy to configure, able to implement in field without needing sophisticated equipment. The method described in this patent addresses all these requirements.